Field
Various communication systems may benefit from jointly designing the precoder and receivers for multi-user multiple-input and multiple-output (MU-MIMO). For example, machine-type communication in long term evolution (LTE) communication systems may benefit from such devices. Such devices may take into account imperfect channel state information (CSI) and joint optimization of transmitters and receivers.
Description of the Related Art
Generally, MU-MIMO is a technique for achieving higher multiplexing gain in wireless communications systems. MU-MIMO has been supported in 3GPP LTE since Rel-8. Optimal sum-capacity for MU-MIMO systems conventionally requires computationally intensive processing, which is not feasible for practical wireless systems, such as, for example, systems based on dirty-paper coding (DPC). An example of such systems is described in M.H.M. Costa, “Writing on dirty paper,” IEEE Transactions on Information Theory, vol. 29, no. 3, pp. 439-441, May 1983, the contents of which are incorporated herein in their entirety. Hence, linear precoding can be the de facto standard scheme for the suppression of multi-user interference (MUI) in MU-MIMO transmissions.
As described in, for example, F. Boccardi, et al., “Precoding schemes for the MIMO-GBC,” in Proc. Int. Zurich Seminar on Communications, Zurich, 2006, pp. 10-13 (the contents of which are incorporated herein in their entirety), although linear precoders do not have the same rate/power efficiency ratio as the DPC-based schemes, they can still achieve the same multiplexing gain. For example, zero-forcing precoders may cancel the MUI entirely under the assumption of perfect CSI, after which the signal-to-noise ratio (SNR) of the MUI-free system can be maximized by block diagonalization in case of multi-antenna terminals. Other linear precoders may maximize the signal-to-interference plus noise ratio (SINR) based on the maximum signal-to-leakage plus noise ratio or the minimum mean square error (MMSE) criterion.
The assumption of perfect CSI is not realistic in wireless systems, and hence residual MUI may always be present. Thus, it may be beneficial to address the problem of jointly designing precoder and receiver under the assumption of imperfect CSI, such that it may be possible to address both MUI minimization and maximization of the gain of the useful signal for each receiver.
MU-MIMO may be generally applied as a technique for wireless communications systems. Linear precoding and block diagonalization from various literatures may also be generally applied. There is presently no practical technique or signaling procedure supporting joint optimization of the transmitter and receiver under imperfect CSI, particularly in high SNR environments, in which the signaling overhead is relatively tolerable.